tsung is a distributed load testing tool.
It is protocol-independent and can currently be used to stress and
benchmark HTTP, Jabber/XMPP, PostgreSQL, MySQL and LDAP servers.
It simulates user behaviour using an XML description file, reports
many measurements in real time (statistics can be customized with
transactions, and graphics generated using gnuplot).
For HTTP, it supports 1.0 and 1.1, has a proxy mode to record
sessions, supports GET and POST methods, Cookies, and Basic
WWW-authentication. It also has support for SSL.

More information is available at http://tsung.erlang-projects.org/ .